This invention relates generally to apparatus and methods for cutting soil and constructing subsurface containment barriers in place. Although not necessarily limited to the following, this invention has particular application in simultaneously cutting through a subsurface volume of soil and emplacing a cement slurry to construct in situ a continuous subsurface wall, horizontal panel or basin around and under a hazardous waste site or contaminated land area. The invention generally may be applied in sites of wide-ranging size, from small sites with buried substances or objects that are too hazardous for excavation to large multi-acre sites having contaminated soils. Preferably, encapsulation may take place without drilling into contaminated soils.
There are many automated ways of cutting or excavating soil (soil as used herein refers to any ground or subsurface material to be cut or excavated). For example, there are scooping devices such as backhoes and clamshells; there are drilling devices such as augers; and there are blasting devices such as dynamite and high pressure fluid. Of particular interest to the present invention as used in the aforementioned exemplary application, however, are the devices and techniques used for cutting soil in the environmental remediation industry.
In the environmental remediation industry it is often desirable to form an impermeable underground containment wall to contain contaminants which are present in the soil and water, thereby preventing or impeding further migration of the contaminants. Hazardous waste sites frequently contain hundreds of thousands of cubic yards of materials which represent a long term threat to ground water quality. While on site treatment is a preferred means of eliminating this threat, this is not always feasible. At some sites the cost of physically removing the material and placing an impermeable liner in the vacated cavity is beyond the resources of the site owner. Sites with buried drums, radioactive dusts, or other airborne hazards may become much more dangerous if excavated. There are also cases where vast and deep areas are only slightly contaminated and require only a containment action. Existing containment technologies provide the means to place a wall around the perimeter of a site or to place a cap over a site.
One common method of constructing a side containment wall is by slurry trenching. This method digs a trench and emplaces a bentonite (clay) slurry as the trenching proceeds. Once the trench is dug, the slurry is replaced with concrete or bentonite modified clay. This technique tends to be slow and very costly at depths exceeding 40 feet. This technique is also limited to forming a relatively wide (e.g., 36 inches) wall even though it is only the thin filter cake build up on the wall that acts as a permeability barrier. The difficulties and expense of forming and ensuring that a continuous wall has been formed increases dramatically below a 40 foot depth, a depth below which this type of wall often needs to extend.
Hydraulic soil cutting using jet grouting is another technique used in the environmental remediation industry. Although this is a useful technique, it is not particularly efficient because much of the jet energy is wasted in passing through fluid before impacting the soil. This causes low production rates, and the cost of the process tends to be higher than for mechanical methods. In most forms of jet grouting it is also difficult to verify that a continuous wall has been formed because the wall is formed from a series of overlapping columns rather than in a continuous fashion. This makes it difficult to form containment walls deeper than 40 feet using this technique.
For forming deeper walls, a four-auger drill system and a clamshell digging tool have been used. The four-auger system is very expensive and slow, capable of forming only 20 to 30 linear feet of wall per day. Clamshell excavating techniques are also very slow.
The foregoing techniques typically provide vertical walls. They do not typically provide bottom barriers under the site, but rather they rely on having a natural layer of low permeability soil (e.g., impermeable rock or clay) underlying the waste site to complete the containment envelope. We are, however, aware of two prior ways of creating an underlying barrier.
Jet grouting technology as practiced by Halliburton Services of Duncan, Okla. allows a bottom to be installed by drilling vertical holes and using the jet grouting process to form overlapping disks of treated material at the bottom elevation. Just as with side wall jet grouting referred to above, it is difficult to verify the integrity of the resulting underlying barrier. Another technique uses horizontal drilled holes with liquid nitrogen freezing. This has quality control problems and requires continuous maintenance. Near surface horizontal pancake fracturing or "block heaving" is another technique which seems to work, but it is difficult to control quality with this technique.
For very large sites containing enormous volumes of waste such as are found in the mining industry for example, the primary, if not the only, suitable technique of waste containment of which we are aware is to physically move the waste onto a synthetic liner and place a cap over it. This has detrimental cost and environmental impact shortcomings as referred to above.
Although the foregoing techniques may be effective in particular applications, they have at least the shortcomings noted above. What is lacking is a cost effective technique for cutting soil to facilitate at least the deep construction of contaminated soil impoundment walls and subsurface containment barriers having high structural integrity around and under waste sites without moving the waste.
The present invention overcomes the above-noted and other shortcomings of the prior art by providing a novel and improved apparatus and method for cutting soil for in situ construction of impoundment walls and/or subsurface containment barriers. The apparatus and methods enable the faster, more efficient and more economical construction of subsurface walls, such as contaminated soil impoundment walls and containment barriers which can extend well below 40 feet into the earth.
In a preferred embodiment, the present invention utilizes both hydraulic and mechanical excavation techniques, but either one can be used alone. This preferred embodiment includes a long beam that is joined by a hydraulic reciprocating member to a pivot joint on the frame of a crane. Within the beam is a tubular conduit which conveys high pressure slurry from an external mixing/pumping unit. At least a portion of the conduit has a plurality of small holes or jet ports which direct the energy of the high pressure slurry toward the face of the soil to be cut. In this particular embodiment the conduit is reciprocated lengthwise so that the jets of slurry contact all the soil in the path of each stroke.
The beam of this preferred embodiment is dense enough so that it is not buoyant in any fluid or loose mixture it might encounter. Accordingly, as each stroke of the conduit is completed, the conduit's weight causes it to sink or fall downward and forward to position itself automatically for the next cut. As this occurs, the crane moves along the ground so that the advancing conduit is pulled through an extended volume of soil which is cut as the apparatus advances. These actions maintain the jets positioned right at the face of the soil to be cut; therefore, the pressurized fluid exiting the jets does not have to pass through much if any intervening fluid before it impacts the soil. Thus, little energy is lost prior to impacting the soil.
In a preferred embodiment for forming a containment barrier, the present invention uses reciprocating high pressure jets of hardening fluid to cut through the soil along a path from one side of a waste site to another without passing through the waste material itself. As the fluid cuts the soil, it also mixes with the soil and subsequently hardens; thus, the high pressure jets, or jet streams, provide both the necessary energy and material for disrupting the soil and forming the barrier. The path traversed by the reciprocated jets is moved transversely so that they pass under the site from one end of the site to the other. As a result, an impermeable containment barrier sheet in the nature of a basin is formed in situ both around and under the waste site. The resulting barrier should have high structural integrity because it is formed in a continuous manner. This technique should be cost effective for constructing in situ surface barriers or partial containment barriers which prevent underground contamination in moving in a particular direction.
The present invention also provides a method of cutting soil, comprising: generating cutting action along an extended locus of soil; and advancing the cutting action along a descending locus of the soil in response to gravity. Generating cutting action can include individually or in combination pumping a fluid through a conduit having a plurality of ports through which the fluid is ejected into the soil adjacent which the conduit is disposed, reciprocating the conduit while pumping the fluid, or reciprocating along the extended locus of soil a beam supporting the conduit. The method can also comprise advancing the cutting action horizontally from the descending locus.
The apparatus of the present invention can be used for constructing a subsurface basin in soil. This apparatus comprises means for creating in situ a continuous cross-sectional portion of the subsurface basin. The means includes a conduit adapted to be disposed in the soil along at least a portion of a locus extending into the soil from two locations at the upper surface of the soil and lying across a cross-sectional area of the basin. The conduit has at least one opening for ejecting fluid under pressure into the soil. The apparatus further comprises means for moving the conduit transversely to the locus.
The present invention provides an apparatus particularly suitable for constructing a containment barrier around and under a waste site disposed in soil, which apparatus comprises: means for cutting a continuous elongate trench through the soil under the waste site and preferably from one side of the waste site to another side of the waste site without intersecting the waste site; means for displacing the means for cutting through the soil so that the elongate trench is extended transversely to itself across a continuum along and under the waste site; and means for placing a barrier material in the transversely extended elongate trench.
The present invention also provides a method of constructing a subsurface barrier, which method comprises: (a) cutting into soil along a continuous locus extending into the soil from two locations on the surface of the soil; (b) simultaneous with step (a), emplacing a fluidized barrier material in the cut soil; and (c) repeating steps (a) and (b) throughout a continuum between a first such locus and a second such locus.
Therefore, from the foregoing, it is a general object of the present invention to provide a novel and improved apparatus and method for cutting soil for constructing in situ impoundment walls and containment barriers. Other and further objects, features and advantages of the present invention will be readily apparent to those skilled in the art when the following description of the preferred embodiments is read in conjunction with the accompanying drawings.